So-called zoom or pancratic objectives of this type generally consist of two distinct lens groups, namely a varifocal front group (often designed as a separate, usually afocal attachment) and a fixed-focus rear or relay group. The varifocal front group comprises a plurality of components including a front component, considered stationary though it may be limitedly axially shiftable for focusing purposes, and at least two axially movable components. The more forwardly positioned movable component or variator, whose refractivity type is generally opposite that of the stationary front component, is linearly shiftable throughout the varifocal range whereas the other movable component, of the same refractivity type, is designed to compensate for excursions of the image plane of the objective due to the shifting of the variator. In order to maintain that image plane in a predetermined position (referred to hereinafter as a zero position) coincident with a receiving surface, such as that of a photosensitive film, the second movable component must generally carry out a nonlinear shift throughout the varifocal range. Thus, conventional objectives of this type have to be provided with accurately machined cam tracks controlling the shifts of these movable components relative to the remaining lenses.
Optical systems are known in which the concurrent parallel shifting of two nonadjacent components of a varifocal group, having the same refractivity type, results in a nearly stationary image plane with three or four zero crossings; see, for example, U.S. Pat. Nos. 2,778,272 and 3,051,052. The varifocal range of such a system is more or less limited to the region extending from the first to the last zero crossing since the deviation of the image plane increases rapidly beyond these points.
U.S. Pat. No. 3,192,829 discloses an optical system in which the second and fourth components of a varifocal group are ganged for concurrent linear displacement while the interposed third component, of opposite refractivity type, carries out a nonlinear motion in order to compensate for the resulting excursions of the image plane. It has also been proposed (see, for example, German published specification No. 2,437,652) to drive the shiftable lens members of a varifocal group by means of a common leadscrew threads of one pitch for an axially displaceable variator component and threads of a different pitch for the compensator component. A difficulty encountered with a system of the latter type is that, for proper stabilization of the image-plane position, the compensator must move only over short distances so that a step-down linkage is required between that component and the leadscrew.